Dynamic charge compression ignition engine with multiple aftertreatment systems

ABSTRACT

Methods, devices, controllers, and algorithms are described for operating an internal combustion engine wherein at least some firing opportunities utilize low temperature gasoline combustion (LTGC). Other firing opportunities may be skipped or utilize some other type of combustion, such as spark ignition. The nature of any particular firing opportunity is dynamically determined during engine operation, often on a firing opportunity by firing opportunity basis. Firings that utilize LTGC produce little, if any, nitrous oxides in the exhaust stream and thus, in some implementations, may require no aftertreatment system to remove them from the exhaust stream.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. application Ser. No.16/021,532, filed on Jun. 28, 2018 which claims priority of U.S.Provisional Patent Application No. 62/528,352, entitled “Dynamic ChargeCompression Ignition Engine”, filed Jul. 3, 2017, both of which areincorporated by reference herein for all purposes.

This application is related to U.S. application Ser. No. 15/485,000filed Apr. 11, 2017 (now U.S. Pat. No. 10,072,592). Application Ser. No.15/485,000 is a Continuation of U.S. application Ser. No. 15/274,029(now U.S. Pat. No. 9,689,328) filed Sep. 23, 2016, which is a Divisionalof U.S. application Ser. No. 15/180,332 (now U.S. Pat. No. 9,476,373),filed Jun. 13, 2016. U.S. application Ser. No. 15/180,332 is aDivisional of U.S. application Ser. No. 14/919,011 (now U.S. Pat. No.9,399,964), filed Oct. 21, 2015, which claims priority to U.S.Provisional Patent Application Nos.: 62/077,439, entitled “Multi LevelDynamic Skip Fire,” filed Nov. 10, 2014; 62/117,426, entitled “MultiLevel Dynamic Skip Fire,” filed Feb. 17, 2015; and 62/121,374, entitled“Using Multi-Level Skip Fire,” filed Feb. 26, 2015. All of these relatedapplications are incorporated herein in their entirety for all purposes.

FIELD OF THE INVENTION

The present invention relates generally to dynamic firing levelmodulation engine operation in which some firing opportunities utilizelean-burn, low temperature gasoline combustion and other firingopportunities utilize standard stoichiometric, spark-ignitioncombustion. The present invention also relates to using twoaftertreatment systems, one optimized for lean burn operation and theother optimized for stoichiometric operation.

BACKGROUND

Most vehicles in operation today (and many other devices) are powered byinternal combustion (IC) engines. An internal combustion enginetypically has a reciprocating piston which oscillates within a cylinder.Combustion occurs within the cylinder and the resulting torque istransferred by the piston through a connecting rod to a crankshaft. Fora four-stroke engine, air, and in some cases fuel, is inducted to thecylinder through an intake valve and exhaust combustion gases areexpelled through an exhaust valve. In typical engine operation, thecylinder conditions vary in a cyclic manner, encountering, in order, anintake, compression, expansion, and exhaust stroke in a repeatingpattern. Each repeating pattern may be referred to as a working cycle ofthe cylinder. The working cycles associated with the various enginecylinders are temporally interleaved, so that the expansion strokeassociated with the various cylinders is approximately equally spaced,delivering the smoothest engine operation. Combustion occurring in theexpansion stroke generates the desired torque as well as various exhaustgases.

Fuel efficiency of internal combustion engines can be substantiallyimproved by varying the displacement of the engine in response to thedemanded torque. Full displacement allows for the full torque to beavailable when required, yet using a smaller displacement when fulltorque is not required can significantly reduce pumping losses andimprove thermal efficiency. The most common method today of implementinga variable displacement engine is to deactivate a group of cylinderssubstantially simultaneously. In this approach the intake and exhaustvalves associated with the deactivated cylinders are kept closed and nofuel is injected when it is desired to skip a combustion event. Forexample, an 8-cylinder variable displacement engine may deactivate halfof the cylinders (i.e. 4 cylinders) so that it is operating using onlythe remaining 4 cylinders. Commercially available variable displacementengines available today typically support only two or at most threedisplacements.

Another engine control approach that varies the effective displacementof an engine is referred to as “skip fire” engine control. In general,skip fire engine control contemplates selectively skipping the firing ofcertain cylinders during selected firing opportunities. Thus, aparticular cylinder may be fired or active during one engine cycle andthen may be skipped or passive during the next engine cycle and thenselectively skipped or fired during the next. Skip fire engine operationis distinguished from conventional variable displacement engine controlin which a designated set of cylinders are deactivated substantiallysimultaneously and remain deactivated as long as the engine remains inthe same variable displacement mode. Thus, the sequence of specificcylinders' firings will always be the same for each engine cycle duringoperation in a variable displacement mode (so long as the engine remainsin the same displacement mode), whereas that is often not the caseduring skip fire operation. For example, an 8-cylinder skip firecontrolled engine operating at a firing fraction of ⅓ will after differpatterns of fired and skipped cylinders on successive engine cycles.

In general, skip fire engine operation facilitates finer control of theeffective engine displacement than is possible using a conventionalvariable displacement approach. For example, firing every third cylinderin a 4-cylinder engine would provide an effective displacement of ⅓^(rd)of the full engine displacement, which is a fractional displacement thatis not obtainable by simply deactivating a set of cylinders.Conceptually, virtually any effective displacement can be obtained usingskip fire control, although in practice most implementations restrictoperation to a set of available firing fractions, sequences, orpatterns. The Applicant has filed a number of patents describing variousapproaches to skip fire control. By way of example, U.S. Pat. Nos.8,099,224; 8,464,690; 8,651,091; 8,839,766; 8,869,773; 9,020,735;9,086,020; 9,120,478; 9,175,613; 9,200,575; 9,200,587; 9,291,106;9,399,964, and others describe a variety of engine controllers that makeit practical to operate a wide variety of internal combustion engines ina dynamic skip fire (DSF) operational mode. Each of these patents isincorporated herein by reference. Many of these patents relate to DSFcontrol in which firing decisions regarding whether to skip or fire aparticular cylinder during a particular working cycle are made in realtime—often just briefly before the working cycle begins and often on anindividual cylinder firing opportunity by firing opportunity basis.

In some applications, referred to as multi-level dynamic skip fire(mDSF), individual working cycles that are fired may be purposelyoperated at different cylinder outputs levels—that is, usingpurposefully different air charges and corresponding fueling levels. Byway of example, U.S. Pat. No. 9,399,964 describes some such approaches.The individual cylinder control concepts used in dynamic skip fire canalso be applied to dynamic multi-charge level engine operation in whichall cylinders are fired, but individual working cycles are purposelyoperated at different cylinder output levels. Dynamic skip fire anddynamic multi-charge level engine operation may collectively beconsidered different types of dynamic firing level modulation engineoperation in which the output of each working cycle (e.g., skip/fire,high/low, skip/high/low, etc.) is dynamically determined duringoperation of the engine, typically on an individual cylinder workingcycle by working cycle (firing opportunity by firing opportunity) basis.It should be appreciated that dynamic firing level engine operation isdifferent than conventional variable displacement in which when theengine enters a reduced displacement operational state, a defined set ofcylinders are operated in generally the same manner until the enginetransitions to a different operational state.

An internal combustion engine typically operates in a repetitive seriesof working cycle to generate engine torque. The working cycle may becharacterized by the thermodynamic cycle used. The thermodynamic cyclemay be depicted on a pressure-volume diagram and may take many forms.Some exemplary thermodynamic cycles include an Otto cycle, a Millercycle, an Atkinson cycle, and a Diesel cycle. In addition to thethermodynamic cycle used, working cycles may be characterized in otherways, for example, by the in-working chamber temperature, fuel/airstoichiometry, or combustion initiation method. Some working cyclesinitiate combustion using an electrical spark and are referred to asspark ignition (SI) working cycles. These SI working cycles couldinclude Miller or Atkinson cycles, as well as lean burn SI or any typeof working cycle using spark ignition. Some working cycles initiatecombustion using self-heating of gases trapped in the working chamberduring the compression stroke. Some working cycles combust aheterogeneous mixture of air and fuel, while other working cyclesutilize a homogeneous charge. Some working cycles have equal expansionand compression ratios, while other working cycles have physicallydifferent expansion and compression ratios or achieve effectivelydifferent ratios by changing the inducted air charge thru early or lateintake valve closure. The many different working cycles have variousattributes, such as fuel efficiency and operational load range. Thedifferent working cycles may also produce different combustiontemperatures, generate different levels and types of noxious emissionsin their exhaust stream, and operate at different air to fuel ratios.

Although the engines described in the cited prior art work well, thereare continuing efforts to further improve fuel economy in enginesoperating under dynamic firing level modulation control. The presentapplication describes additional features and enhancements that canimprove engine performance in a variety of applications.

SUMMARY

The present invention relates to dynamic firing level modulationcontrol, which includes both dynamic skip fire, multi-level dynamic skipfire, and dynamic multi-charge level engine. In one aspect, a method forcontrolling an engine is described. Selected working cycles are skippedand selected active working cycles are fired to deliver a desired engineoutput. One or more working chambers are capable of generating multiplepossible levels of torque output e.g., for the same cam phaser and/orMAP (intake manifold absolute pressure) settings. A particular level oftorque output (e.g., high or low torque output) is selected for each ofthe fired working chambers. The low torque level output utilizes a lowtemperature gasoline combustion (LTGC) working cycle. The LTGC may be ahomogeneous charge compression ignition (HCCI) working cycle, apartially premixed compression ignition (PPCI) working cycle, or someother type of LTGC working cycle. Various embodiments relate to enginecontrollers, software, and systems that help implement the above method.

In some embodiments, the LTGC working cycle is a homogeneous chargecompression ignition (HCCI) working cycle. HCCI operation may beproduced by either recompression and re-expansion of trapped gas in theworking chamber or by inducting exhaust gas into the working chamberduring its intake stroke. The HCCI working cycles are temporallyinterleaved with skipped and/or some other type of working cycle todeliver the requested engine torque.

In other embodiments, both the high and low torque level firings utilizea LTGC working cycle. Intermediate torque output levels can be obtainedby temporally interleaving low and high torque output firings to delivera requested torque.

Dynamic firing level modulation control may be performed in a widevariety of ways. In some embodiments, for example, decisions regardingwhether to fire or skip each working cycle and/or decisions whether toselect a particular level of torque output for a fired working chamberare performed on a firing opportunity by firing opportunity basis.

In yet another embodiment, two aftertreatment systems may be used withan internal combustion engine capable of operating in bothstoichiometric and lean burn modes. One aftertreatment system is usedfor stoichiometric working cycles, while the second is used for leanburn working cycles.

The various aspects and features described above may be implementedseparately or in any combination.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention and the advantages thereof, may best be understood byreference to the following description taken in conjunction with theaccompanying drawings in which:

FIG. 1 shows an operating region for an HCCI operated enginesuperimposed on an operating region for a naturally-aspirated,throttle-controlled, spark-ignition (SI) operated engine.

FIG. 2 shows representative intake and exhaust valve lift profilesrequired to run SI and HCCI working cycles using recompression in HCCIoperation.

FIG. 3 shows valve lift profiles for an HCCI working cycle obtained by“rebreathing” exhaust gases.

FIG. 4 shows representative specific fuel consumption for SI and HCCIengine operation within their respective operating ranges.

FIG. 5 shows an expanded low load, high efficiency operating regionusing dynamic charge compression ignition (DCCI) in accordance with anon-exclusive embodiment of the present invention.

FIG. 6 shows an efficiency benefit of low-load DCCI operation inaccordance with a non-exclusive embodiment of the present invention.

FIG. 7 shows an expanded mid-high load operating range enabled by DCCIoperation in accordance with a non-exclusive embodiment of the presentinvention.

FIG. 8 shows fuel savings associated with DCCI operation over anengine's full load range in accordance with a non-exclusive embodimentof the present invention.

FIG. 9 shows operating ranges for two distinct, high-efficiency, lowtemperature gasoline combustion (LTGC) regions.

FIG. 10 shows specific fuel consumption for a low load and high loadLTGC working cycle.

FIG. 11 shows DCCI being used to operate over a full load range inaccordance with a non-exclusive embodiment of the present invention.

FIG. 12 shows a DCCI fuel consumption benefit over a full load range inaccordance with a non-exclusive embodiment of the present invention.

FIG. 13 is a plot illustrating fuel efficiency benefits of operating aninternal combustion engine with both homogeneous lean burn combustionand dynamic skip fire engine control in accordance with the presentinvention.

FIG. 14 is a bar chart illustrating improved exhaust temperature controlresulting from homogenous, lean burn combustion with dynamic skip firecontrol of an internal combustion engine in accordance with anon-exclusive embodiment of the present invention.

FIG. 15 shows an internal combustion engine and two aftertreatmentsystems.

FIG. 16 shows a table listing multiple embodiments for operating anengine in both stoichiometric and lean burn modes in accordance with anon-exclusive embodiment of the present invention.

FIG. 17 illustrates a cylinder of an internal combustion engine andfirst and second aftertreatment systems in accordance with the presentinvention.

FIG. 18. illustrates another embodiment of cylinder of an internalcombustion engine and first and second aftertreatment systems inaccordance with the present invention.

FIG. 19 illustrates a flow diagram for controlling operation of aninternal combustion engine in a stoichiometric or lean burn mode inaccordance with a non-exclusive embodiment of the present invention.

In the drawings, like reference numerals are sometimes used to designatelike structural elements. It should also be appreciated that thedepictions in the figures are diagrammatic and not to scale.

DETAILED DESCRIPTION

The present invention relates to methods and systems for operating aninternal combustion engine wherein some firing opportunities in adynamic firing level modulation controlled engine utilize lowtemperature gasoline combustion (LTGC). Other firing opportunities maybe skipped or utilize some other type of combustion, such as sparkignition. The nature of any particular firing opportunity is dynamicallydetermined during engine operation, often on a firing opportunity byfiring opportunity basis. Firings that utilize LTGC produce little, ifany, nitrous oxides (NO_(x)) in combustion and thus require noaftertreatment system to remove them from a combustion exhaust stream.One type of LTGC is homogeneous charge compression ignition (HCCI) wherea homogenous fuel air mixture auto ignites. Typically, HCCI uses a leanair-to-fuel ratio. Other types of LTGC include gasoline directcompression ignition (GDCI), spark assisted compression ignition (SACI),and partially premixed compression ignition (PPCI). In GDCI and PPCIfuel auto ignites as it is injected into a hot air charge caused bycompression of an inducted air charge. In SACI a spark is used tocontrol ignition timing rather than relying on auto-ignition. In PPCI astratified charge is used to manage the fuel burn rate.

Operating an engine with LTGC working cycles is often advantageous. Forexample, operation in HCCI mode is advantageous for at least tworeasons. First, HCCI is more fuel efficient than spark-ignited (SI)combustion for a given cylinder load. HCCI has fuel efficiencyimprovements in the range of 10-18% for light cylinder loads, such ascylinder loads of 1 to 4 bar brake mean effective pressure (BMEP). Thefuel consumption improvements are due to the rapid rate of heat releaseresulting from bulk auto-ignition and unthrottled operation. A secondadvantageous feature of HCCI is that it produces very low levels ofharmful emissions. HCCI typically utilizes an extremely lean or dilutedfuel-to-air mixture and the resulting combustion results in lowcombustion temperatures and complete, or near complete, fuel oxidation.Despite lean operation, exhaust gas NO_(x) levels comfortably meetcertification standards without expensive exhaust aftertreatmentsystems. HCCI can also be implemented and controlled relatively cheaplywith variable valve actuation based on cam actuated intake and exhaustvalves and does necessarily require high pressure fuel injectors orspecial ignition systems.

Unfortunately, HCCI in its simplest and most efficient form is limitedby high rates of pressure rise and combustion noise for heavy loads, andcombustion stability near the lean flammability limit for light loads.For heavy loads NO_(x) emissions may also exceed regulated levelswithout use of a lean burn aftertreatment systems. A typical HCCIoperating region 110 is generally confined to a 1 to 4 bar BMEP rangebetween engine speeds of 800 and 4000 rpm as shown in FIG. 1. This rangeis representative only and HCCI may be operated outside of this rangedepending on engine design details. The HCCI operating region isconstrained for low engine loads by combustion stability and at highengine loads by pressure-rise rates, combustion noise, Extensiveresearch efforts have been expended attempting to expand this region,including intake air boosting, multiple injection strategies,spark-assistance, and dual-fuel operation, among others. However, theseconcepts generally require specialized new hardware and complexcalibrations, which can make the technology prohibitively expensive.

Also shown in FIG. 1 is an operating region 120 for anaturally-aspirated, throttle-controlled, spark-ignition combustionengine, which both covers the HCCI operating region 110 and extends tohigher and lower engine loads. A desirable attribute of SI operation isits large operating range, which meets the varying torque and speeddemands encountered in typical motor vehicle operation. Undesirableaspects of SI operation include a requirement for a three-way catalyst(TWC) aftertreatment system to reduce noxious emissions and relativelylow efficiency as compared to HCCI operation. The current inventionprovides for mixed mode operation, mixing SI (or some other type ofworking cycle) and HCCI (or some other type of LTGC working cycle)operation dynamically, in some cases on a working cycle by working cyclebasis, to provide a requested engine output torque at high fuelefficiency over a larger operating range than can be achieved using HCCIalone.

FIG. 2 shows representative intake and exhaust valve lift profiles forSI and HCCI operation. In FIG. 2 HCCI operation is maintained byrecompression and re-expansion of trapped exhaust gases. Valve liftprofiles 202 and 204 illustrate the exhaust and intake valve profiles,respectively, associated with a SI working cycle. Valve lift profile 212and 214 illustrate the exhaust and intake valve profiles, respectively,associated with a HCCI working cycle. In the SI working cycle the valveshave a higher lift profile and stay open longer than in the HCCI workingcycle. In an SI working cycle both the intake and exhaust valves aretypically open at 0 degrees of crank angle, corresponding to a top deadcenter (TDC) position of a piston in a cylinder (or more generallyworking chamber). In contrast, during HCCI with recompression andre-expansion, the exhaust valve closes very early to retain largeamounts of hot residuals, which help promote auto-ignition for HCCIcombustion. The intake valve opens late resulting in negative valveoverlap in HCCI operation, in contrast to positive valve overlap in SIoperation. During the time interval when both the exhaust and intakevalves are closed the trapped exhaust gas is re-compressed and thenre-expanded, acting as a gas spring. The recompression and re-expansionregions may be generally symmetric around TDC to minimize pumpinglosses. In some embodiments for both SI and HCCI operation, the intakeand exhaust valve timing may be controlled independently, that is thephase of the intake and exhaust valve opening and closing can beindependently adjusted using a cam phaser. A working chamber may beshifted between SI and HCCI operation by using a different cam profile,resulting in a different, non-zero valve lift profile for the two typesof working cycles. It should be appreciated that the air chargeassociated with a SI and HCCI working cycle is generally differentbecause of exhaust gas entrapment in the cylinder during portions of theexhaust and intake strokes when operating in HCCI.

FIG. 3 shows another HCCI control strategy called “rebreathing”. Thevalve profiles 202 and 204 associated with spark ignition operation areas shown in FIG. 2. In HCCI with rebreathing, the exhaust valve vents inthe exhaust stroke as it does during SI operation, profile 202. Duringthe subsequent intake stroke the exhaust valve again opens followingprofile 312. Re-opening the exhaust valve during the intake strokeinducts hot exhaust gases into the cylinder, which help promoteauto-ignition for HCCI combustion. As in HCCI with recompression, boththe intake and exhaust valve timing may be phased independently toprovide precise control over gases flowing into and out of the cylinder.The air charge associated with SI and HCCI with rebreathing aregenerally different because of induction of exhaust gases into thecylinder during the intake stroke during HCCI operation. The presentinvention may use HCCI with recompression or HCCI with rebreathing ormay operate with both control strategies.

For both types of HCCI described above, or a similar LTGC operatingstrategy, excess oxygen in the exhaust stream may be minimized oreliminated by employing high levels of residual dilution of the cylindercharge and/or using an external exhaust gas recirculation system thatintroduces exhaust gases into an intake manifold that supplies air tothe engine's cylinders. That is, the composition of the cylinder chargemay be controlled so that the exhaust gases have little or no residualoxygen. When HCCI operates in such a manner, a three-way catalyst may beused to reduce noxious emissions in the exhaust stream coming from SIfirings that may be interleaved with the HCCI firings.

FIG. 4 shows representative specific fuel consumption for SI and HCCIoperation within their respective operating ranges. Curve 402 depictsthe specific fuel consumption for SI operation over its entire loadrange. Curve 404 depicts the specific fuel consumption over a typicalload range for HCCI operation. Over this operating range the fuelefficiency benefits of HCCI operation are denoted by area 406. In priorart dual-mode SI-HCCI engines, the engine operated in HCCI mode when itis in that operating range and operated in SI mode when the engine loadis above or below the HCCI range. There was a discrete switch in engineoperating mode between SI and HCCI operation at both the low and highload HCCI limits. While these prior art dual-mode operation engine'simproved fuel efficiency somewhat, the fuel efficiency gains werelimited by the relatively narrow HCCI operating window. For clarity, theSI load curve 402 does not extend to zero load, where by definition thespecific fuel consumption asymptotically approaches infinity; however,in practice an SI engine can operate stably under zero and low loadconditions by heavy throttling of the air intake.

As described in Applicant's U.S. Pat. No. 9,399,964, which isincorporated herein by reference, an extension of dynamic skip fire(DSF) control is multi-level dynamic skip fire (mDSF) control. Ingeneral, multi-level dynamic skip fire contemplates the use of one ormore intermediate (lower) firing state(s) such that differentinterspersed fired working cycles may produce different torque outputs.For example, in some embodiments the output of an intermediate firingmay be on the order of 60 to 80% of a full firing charge. In somespecific mDSF systems, a firing opportunity may be executed in threedifferent states; a high charge fire, a low charge fire, or a skip,which provides finer control over the engine torque waveform thanstandard DSF, that does not have multiple discrete firing levels.

One potential advantage of using mDSF or more generally dynamic firinglevel modulation is the potential to attain better fuel efficiency.However, to realize efficiency gains the lower or intermediate outputworking cycles should operate more efficiently than the higher outputworking cycles. In some of the embodiments described in U.S. Pat. No.9,399,964 working cycles with higher efficiency include an Atkinsoncycle or Miller over-expanded cycle. For example, early intake valveclosing (EIVC) and late intake valve closing (LIVC) strategies may beemployed to achieve an effectively longer expansion stroke compared withthe intake stroke. This augments work extraction for a given cylindercharge size and improves efficiency by more than 5% as compared to an SIworking cycle. Based on a federal test protocol (FTP) drive cycle, theestimated fuel economy gains of mDSF over standard DSF are severalpercent.

Dynamic charge compression ignition (DCCI) is an extension of mDSF. InDCCI at least one mDSF firing level operates with a low temperaturegasoline combustion working cycle, such as HCCI. HCCI is an idealcandidate for the low level or partial charge, since it is naturallyrestricted to lower loads. Due to the significantly higher efficiency ofHCCI working cycles, the potential fuel economy gains of DCCI aresignificant. FIG. 5 shows an expanded low load, high efficiencyoperating region using low-load dynamic charge compression ignition(DCCI). By using DCCI, HCCI operation can be extended into the region520, which includes zero load, without special control strategies tomaintain combustion stability. The requested engine torque could beobtained by an engine controller specifying a firing density of HCCIfirings mixed with skipped firing opportunities to deliver the requestedtorque. The HCCI firings may be evenly distributed using a sigma-deltacontroller or some other control strategy as described in incorporatedU.S. Pat. No. 9,399,964 and some of the other previously citedApplicant's prior patents. The skipped working cycles may eitherdeactivate the cylinder, so that no air is pumped through the cylinder,or simply skip fueling the cylinder while still pumping air. The latterstrategy does not require valve deactivation hardware and thus may beless costly to implement. Since the working cycles are either skipped oruse low temperature combustion, no special aftertreatment may berequired in some embodiments. The engine controller may vary the chargeassociated with the HCCI firing and the firing density to maximize fuelefficiency and obtain acceptable noise, vibration, and harshness (NVH)performance

An operating point that may be enabled by DCCI operation is engine idle,which generally occurs at engine speeds near 800 rpm and requires nooutput engine torque. Running all cylinders in HCCI mode produces moretorque than required for idle without using special and potentiallyinefficient combustion control strategies. Using DCCI allows somecylinders to operate in an efficient HCCI regime, while other cylindersare skipped, so that the overall engine torque matches idlerequirements.

FIG. 6 shows an efficiency benefit of low-load DCCI operation. Theregion 406 is the same as shown in FIG. 4, which illustrates the benefitof dual-mode, SI/HCCI operation. By using DCCI additional fuel economyis obtained as denoted by region 604 lying between the HCCI and DCCIcurve. While not shown in FIG. 6 region 604 extends to zero load andoffers fuel savings over SI operation in the load range below the pureHCCI operating load range. In this embodiment, the engine control mayrevert to all-cylinder, SI operation for loads greater than the highload HCCI boundary. Operation in low-load DCCI generates little or nonoxious emissions and thus no aftertreatment system is required whileoperating in this mode. A conventional three-way catalyst can clean upthe exhaust gases when some or all cylinders are operating in SI mode,which may occur for loads above the HCCI load range. Rebalancing of theoxidation/reduction balance in the three-way catalyst, by running with arich air/fuel ratio or injecting hydrocarbons into the catalyst, may berequired when transitioning to SI operation.

FIG. 7 shows an expanded high-efficiency operating range enabled by fullDCCI, including the mid-high load region 702. In this mid-high loadregion 702, full DCCI would switch between SI working cycles, HCCIworking cycles, and potentially skipped working cycles. The SI workingcycles would potentially generate high NO_(x) emissions andaftertreatment would be required in this operating region. A specialaftertreatment system capable of removing NO_(x) in a lean exhauststream may be required. In other embodiments, the HCCI working cyclesmay operate with high dilution levels near a stoichiometric air/fuelratio and thus a standard three-way catalyst may be used.

FIG. 8 shows the specific fuel consumption benefit associated withoperating DCCI in the mid-high load region 702. Region 804 lying betweenthe SI curve and DCCI curve in the load region above the HCCI operationlimit denotes the fuel economy gain associated with this mode ofoperation. In this embodiment, DCCI blends standard SI, HCCI, andpotentially cylinder skips to provide extended regions of highefficiency operation and a smooth SI-HCCI mode transition. Operation inthe low load region for loads in and smaller than the HCCI load rangemay be as described before relative to FIGS. 5 and 6. Loads in thisregion may also be obtained by mixing SI, HCCI, and skipped workingcycles. This may be advantageous if the resulting firing patternprovides improved fuel efficiency with acceptable NVH as compared toHCCI only operation.

Modeling indicates that full DCCI can potentially double the fuelefficiency benefit of mDSF using a Miller or Atkinson cycle as thelow-level firing relative to standard DSF for a 2.0-L, 4-cylinder engineoperated over a representative drive cycle. Full DCCI may employ HCCIover a relative narrow operating range, the base range 110 shown in FIG.1 or an even smaller range, where HCCI operates robustly with minimalchanges in engine hardware. Its implementation cost can be significantlylower than other advanced combustion strategies currently underinvestigation. Aftertreatment with a conventional three-way catalystcould be a challenge if running mixed mode HCCI/SI where the combinedexhaust stream will likely be fuel-lean. Some embodiments may utilize aNO_(x) trap or selective catalyst reduction to reduce NO_(x) emissionsin an oxygen rich exhaust. In other embodiments, HCCI may be operatedover a more limited range with appropriate exhaust gas dilution suchthat excess oxygen levels in the exhaust stream are minimized. In eithercase a DCCI system provides a cost effective path to improve fueleconomy.

FIG. 9 shows operating ranges for two distinct LTGC operating regions.The mid-low load region, LTGC #1, is denoted as region 910. The mid-highload region, LTGC #2, is denoted as region 920. HCCI is a subset of LTGCand could correspond to LTGC #1, region 910. Other types of lowtemperature gasoline combustion working cycles may operate in themid-high load region 920. These cycles include, but are not limited to,spark assisted compression ignition (SACI), partial premixed compressionignition (PPCI) and boosted HCCI. If the cycle is boosted, that is theair pressure in the intake manifold is above atmospheric pressure, theair pressure boost may be achieved using either a turbocharger orsupercharger. Both LTGC #1 and LTGC #2 are only viable within certainconstrained operating ranges, region 910 and 920, respectively, and thuswould not provide the full dynamic coverage required for motor vehicleoperation. By mixing firings between the two regions, the intermediateload region 930 may be covered. By mixing firings from region 910, andperhaps region 920, with skips, the low load region 940 may be covered.

FIG. 10 shows representative specific fuel consumption of LTGC #1 andLTGC #2 working cycles. Curve 1010 may be the specific fuel consumptionversus load for HCCI operation. Curve 1020 is the specific fuelconsumption for some form of a LTCG working cycle that works at highercylinder loads than curve 1010. Both of these working cycles may providesignificant benefits, such as improved fuel economy and lower NO_(x)emissions, as compared to SI working cycle operation. There are regionswhere each of the working cycles alone cannot cover.

By mixing LTGC #1, LTGC #2, and skipped working cycles, completecoverage over the entire engine load range may be obtained as shown inFIG. 11. FIG. 11 shows DCCI being use to both expand the high efficiencyLTGC #1 region to zero load and also to bridge the gap between the twoLTGC operating strategies shown in FIG. 10. Each cylinder couldpotentially switch between LTGC #1, LTGC #2 and skipping based on thedesired torque level.

FIG. 12 shows the DCCI fuel consumption benefit, which includes anexpanded low load range and a bridged gap where originally there was nocoverage. By switching among LTGC #1, LTGC #2 and skipping, it ispossible to bridge the coverage gaps and provide an even larger fuelconsumption reduction compared to operation with LTGC #1 alone.

Dynamic firing level modulation controllers suitable for determiningwhich working cycles to skip, fire at a high output level, and fire at alower or intermediate level during DCCI operation are described in U.S.Pat. Nos. 9,689,328; 9,476,373; 8,099,224 (each of which is incorporatedherein by reference) and other of Applicant's patents and patentapplications.

In some preferred embodiments, the firing level decisions are made on afiring opportunity by firing opportunity basis although, that is not arequirement. In some embodiments, for example, the determination of athen current desired effective firing fraction and the determination ofthe appropriate firing level (e.g., high, low, skip, etc.) for the nextdetermined working cycle are make on a firing opportunity by firingopportunity basis. An advantage of firing opportunity by firingopportunity control is that it makes the engine very responsive tochanged inputs and/or conditions. Although firing opportunity by firingopportunity determination of the firing sequence is very effective, itshould be appreciated that the firing decisions can be refreshed moreslowly while still providing good control (e.g., the firingfraction/sequence determinations may be performed every revolution ofthe crankshaft, every two or more firing opportunities, etc.).

Various implementations of the invention are very well suited for use inconjunction with dynamic firing level modulation operation in which anaccumulator or other mechanism tracks the portion of a firing that hasbeen requested, but not delivered, or that has been delivered, but notrequested such that firing decisions may be made on a firing opportunityby firing opportunity basis. However the described techniques areequally well suited for use in virtually any firing level modulationapplication including operation using fixed firing patterns or firingsequences. Similar techniques may also be used in conjunction withvariable stroke engine control in which the number of strokes in eachworking cycle are altered to effectively vary the displacement of anengine.

The described approaches are particularly well suited for use in dynamicfiring level modulation engine operation in which some firingopportunities utilize low temperature combustion. Low temperaturecombustion has an advantage of being more efficient than many othertypes of working cycles. It also has an advantage in producing nearcomplete fuel oxidation and little NO_(x) generation, such that noaftertreatment system is required in some cases.

Simulated Test Results

Simulated test results of running an internal combustion engine in alean burn mode with dynamic skip fire (DSF) demonstrate a number ofsynergistic benefits.

One such benefit is that the useful operating range of lean burn expandsto lower torque levels with DSF than otherwise possible with simply leanburn. This benefit is highlighted in a comparison of engine mapsdepicted in FIG. 1 versus FIG. 5.

In FIG. 1, the lean burn (e.g., base HCCI) operating mode is limited toa narrow torque (engine load) bandwidth 110 over a wide range of enginespeeds. Standard SI combustion is needed if the demanded torque iseither above or below this range.

In contrast, the engine may operate in the lean burn mode at low torquedemands with DSF. FIG. 5 shows an expanded lean burn (e.g., low loadDCCI) region at low torque loads replacing standard SI combustion. WhenDSF and lean burn combustion are combined, an air-fuel ratio greaterthan 1.0 can be used at lower normalized torque values than previouslypossible. As a result, improved fuel consumption is realized under lowtorque conditions.

Referring to FIG. 13, a plot 1300 further illustrating the synergisticbenefit of operating an internal combustion engine using both lean burncombustion and DSF is shown. In this plot, Net Specific Fuel Consumption(NSFC) values are provided along the vertical axis, while normalizedtorque values (NMEP) are provided along the horizontal axis. The plot1300 has four curves. Curve 1310 represents baseline operation of astoichiometrically fueled engine operating on all cylinders. Torquegeneration is controlled by a throttle that can reduce air pressure inan intake manifold that feeds air into the engine reducing an inductedair charge. Curve 1312 represents stoichiometric combustion with dynamicskip fire control. Here torque control is primarily achieved by changingthe firing density of the cylinders of the engine. Curve 1314 representshomogeneous, lean burn combustion on all engine cylinders. This curvecoincides with curve 1310 in the low torque and high torque regions.Overlap occurs because at low torques the combustion needs to bestoichiometric to maintain combustion stability. At high torques,stoichiometric combustion is required to generate the necessary power.In the intervening region, between approximately 3 and 14 bar NMEP,operation with homogeneous lean burn combustion offers significant fuelsavings as compared to stoichiometric operation. Curve 1316 representshomogeneous, lean burn combustion with the dynamic skip fire control.This curve overlaps with curve 1314, except for low torques belowapproximately 5 bar NMEP. Here the low torques are achieved by skippingfiring opportunities, so combustion stability may be maintained on thefiring cylinders.

Region 1302 illustrates the improved fuel efficiency obtained withstoichiometric combustion and DSF control as compared to all cylinderoperation with stoichiometric combustion. The region 1304 illustratesthe improved fuel efficiency obtained with all cylinder homogeneous leanburn combustion as compared to all cylinder stoichiometric combustion.The region 1306 illustrates the improved fuel efficiency obtained by DSFcontrol with homogenous lean burn combustion as compared tostoichiometric. Combining DSF control with homogeneous lean burncombustion maximizes fuel economy for low torque outputs.

Another advantage of combining DSF control with homogenous lean burncombustion is that engine exhaust gas temperature can be raised andbetter controlled. Referring to FIG. 14, a chart 1400 showing testresults demonstrating improved exhaust temperature control is shown.Along the horizontal axis, upstream TWC temperatures are provided inCelsius (C) as measured by an exhaust gas temperature gauge. Thevertical axis provides a range of consumed air-fuel mass values.

In the chart 1400, the medium shaded bars 1402 provide exhausttemperatures at various consumed air-fuel mass values for all cylinderhomogeneous lean burn operation (denoted in the figure as Lean).

The light shaded bars 1404 provide exhaust temperatures at variousconsumed air-fuel mass values for homogeneous, lean burn combustion withDSF control (denoted in the figure as λDSF).

The dark shaded bars 1406 are provided where the bars 1402, 1404overlap. When a light shaded bar 1404 is higher than the correspondingdark shaded bar 1406, the height of the dark shaded bar equals themedium shaded bar 1402 (i.e., the upstream TWC temperatures for λDSF isgreater than all cylinder lean burn). When the medium shaded bar 1402 ishigher than the dark shaded bar 1406, the height of the dark shaded bar1406 equals the light shaded bar 1404 (e.g., the upstream TWCtemperatures for all cylinder lean burn is greater than λDSF).

The chart 1400 thus shows the exhaust temperature distribution for λDSFis both narrower and higher than all cylinder operation, which generallyimproves aftertreatment system efficacy reducing harmful emissions.

Multiple Aftertreatment Systems for Different Combustion Modes

The various modes of engine operation noted above have advantages anddisadvantages. For example:

(1) Throttle-controlled, spark-ignition or “SI” combustion engines canoperate over a wide range of high torque and engine speed demands Theseengines may be either naturally aspirated or may be boosted with theinducted air above atmospheric pressure. SI type combustion engines canoperate over this wide range at a stoichiometric air/fuel ratio. Ifoperated with a stoichiometric air/fuel ratio a three-way catalyticconverter type aftertreatment system may be used. For lean air/fuelstoichiometries, other types of aftertreatment systems are generallyrequired. A limitation of three-way catalytic converter aftertreatmentsis that they require, on average, a stoichiometric air/fuel ratio andthus are limited to use with SI engines.

(2) For low load DCCI operation (e.g., FIG. 5), little to no noxiousemissions are generated, and as a result, no aftertreatment system maybe needed. However, at expanded high load DCCI operation (e.g., FIG. 7),high levels of NO_(x) are potentially generated, requiring anaftertreatment system capable of reducing NO_(x) emissions.

(3) HCCI at low torque loads provides the benefits of high fuelefficiency and low levels of harmful admissions. At higher torque loadshowever, NO_(x) emissions become excessive, requiring an aftertreatmentsystem capable of reducing NO_(x) emissions.

With an internal combustion engine capable of operating in multiplemodes, a single aftertreatment system may be inadequate. Using twoaftertreatment systems, one for SI stoichiometric operation and theother for lean burn operation, would be advantageous.

Referring to FIG. 15, an internal combustion engine 2000, a firstexhaust system 2020 with a first aftertreatment system 2002 and a secondexhaust system 2022 with a second aftertreatment system 2004 is shown.The internal combustion engine 2000 includes four (4) cylinders 2006. Invarious embodiments, each of the four cylinders 2006 can be configuredto either operate (a) in only a stoichiometric mode, (b) in only a leanburn mode, or (c) selectively in both stoichiometric and lean burnmodes.

Depending on a cylinder's configuration, it exhausts to either the firstexhaust system 2020, the second exhaust system 2022, or both the firstand second exhaust systems 2020 and 2022. The output of the two exhaustsystems 2020, 2020 exhaust to the atmosphere after the exhaust passesthrough either the first aftertreatment system 2002 or the secondaftertreatment system 2004 respectively. The outputs of the two exhaustsystems 2020 and 2022 may be joined prior to reaching the atmosphere ormay individually exhaust to the atmosphere as shown in FIG. 15.

The stoichiometric modes may include conventional spark-ignited (SI)operation, and under certain operating conditions, LTGC operation aswell. With LTGC, stoichiometric operation can be achieved bysubstituting large amounts of oxygen depleted exhaust or residualgasses, such as generated by Exhaust Gas Recirculation (EGR), instead ofair into the cylinders. As a result, O₂ levels remain sufficiently lowfor TWC aftertreatment systems, making LTGC possible in thestoichiometric mode. In addition, Otto, Miller and Atkinson are eachexamples of thermodynamic cycles that generally, although notexclusively, use stoichiometric SI combustion.

The lean burn modes may include HCCI operation, LTGC operation, GDCIoperation, SACI operation, PPCI operation, and Diesel operation. Withlean burn Diesel operation, certain cylinders can be operated in apremixed and/or partially premixed burn, resulting in cleaner and moreefficient operation at lower loads. In addition, lean burn variations ofMiller and Atkinson cycles are also considered examples of lean burnmodes of operation.

As described above in certain load and speed ranges some cylinders of anengine may be operating in a stoichiometric mode, while other cylindersare operating in a lean burn mode. Under other circumstances, somecylinders may be skipped as well.

Referring to FIG. 16, a table 1600 showing various non-exclusiveembodiments of operating the four-cylinder internal combustion engine2000 in various combinations of stoichiometric (“S”) and lean burn (“L”)modes is illustrated. Each embodiment is designated A through E in theleft column. Within the row for each embodiment, the “S” or “L” mode ofoperation for each of the four cylinders (CYL 1, CYL 2, CYL 3 and CYL 4)is provided. For instance:

With embodiment A, the four cylinders CYL 1, CYL 2, CYL 3 and CYL 4 aredesignated to operate in the L, S, S and L modes respectfully;

With embodiment B, the four cylinders CYL 1, CYL 2, CYL 3 and CYL 4 aredesignated to operate in the S, S+L, S+L and S modes respectfully;

With embodiment C, the four cylinders CYL 1, CYL 2, CYL 3 and CYL 4 aredesignated to operate in the S+L, S+L, S+L and S+L modes respectfully;

With embodiment D, the four cylinders CYL 1, CYL 2, CYL 3 and CYL 4 aredesignated to operate in the S+L, S, S and S+L modes respectfully; and

With embodiment E, the four cylinders CYL 1, CYL 2, CYL 3 and CYL 4 aredesignated to operate in the L+S, L, L and L+S modes respectfully.

In some embodiments, the cylinders are organized into two groups; onefor stoichiometric operation only and the other for lean burn operationonly (e.g., embodiment A).

In other embodiments, all the cylinders can operate in both thestoichiometric and lean burn modes (e.g., embodiment C).

In yet other embodiments, the cylinders are organized into one group ofcylinders operating in a specific mode (lean burn or stoichiometric) andthe other group capable of operating in both modes (e.g., embodiments B,D, and E).

It should be understood that the embodiments provided in FIG. 16 aremerely illustrative and should be in no way considered limiting. On thecontrary, any possible combination of stoichiometric and lean burnoperation may be used by an internal combustion engine, regardless ofthe number of cylinders. For instance, with internal combustion engineshaving 1, 2, 4, 6, 8, 12 or more cylinders, one or more cylinders can beconfigured to operate in (a) only the stoichiometric mode, (b) only thelean burn mode or (c) both modes. As such, the number of possiblecombinations are too numerous to exhaustively list herein.

Referring to FIG. 17, a drawing 1700 illustrating an engine controller2102 selectively coupling the exhaust of a representative cylinder 2006of the internal combustion engine 2000 is shown. Similar to many of theembodiments of FIG. 16, the one cylinder 2006 is capable of operation ineither a stoichiometric mode or a lean burn mode. The cylinder 2006includes two exhaust valves 2104A and 2104B. The first exhaust valve2104A, when opened, provides a passageway to the first exhaust system2020, which directs exhaust gases thru first aftertreatment system 2002.The second exhaust valve 2104B, when opened, provides a passageway tothe second exhaust system 2022, which directs exhaust gases thru secondaftertreatment system 2004.

In the non-exclusive embodiment shown, the first aftertreatment system2002 is configured for a first mode of operation. The secondaftertreatment system 2004 is configured for a second mode of operation.For example, the first aftertreatment system 2002 may be optimized forstoichiometric operation, while the second aftertreatment system 2004may be optimized for lean burn operation. In other embodiments, thefirst aftertreatment system 2002 may be configured for stoichiometricoperation and the second aftertreatment system 2004 configured for bothstoichiometric and lean burn operation.

During operation of the internal combustion engine 2000, the enginecontroller 2102 controls the two exhaust valves 2104A and 2104B of eachof the cylinders 2006. When a working cycle of a cylinder isstoichiometric, the engine controller 2102 opens its exhaust valve2104A, while closing the exhaust valve 2104B. As a result, combustiongases and any combustion particulates are provided a pathway to andexhausted through the first aftertreatment system 2002. Alternatively,when the working cycle of a cylinder 2006 is lean burn, the enginecontroller 2102 opens the exhaust valve 2104B, while closing the exhaustvalve 2104A. As a result, combustion gases and any combustionparticulates pass and are exhausted through the second aftertreatmentsystem 2004.

It should understood that FIG. 17 shows all cylinders capable ofexhausting into both exhaust systems, but this is not a requirement. Insome embodiments of the internal combustion engine 2000, certaincylinders may be capable of operating only in either only thestoichiometric or lean burn mode and thus exhaust only into theappropriate exhaust system for that combustion mode. In these cases,only a single exhaust valve is required; however, it still may beadvantageous to provide two exhaust valves that exhaust into a commonaftertreatment system.

Referring to FIG. 18, a diagram 1800 shows additional detail of anembodiment of an engine with two distinct aftertreatment systems. Inthis embodiment, the first aftertreatment system 2002 may be configuredfor operation with exhaust products from nominally stoichiometriccombustion, while the second aftertreatment system 2004 is nominallyused for both stoichiometric and lean burn combustion.

If cylinder 2006 operates in a stoichiometric combustion mode, thecombustion exhaust products are exhausted into the first aftertreatmentsystem 2002 via the exhaust valve 2104A and a first exhaust system 2020.The first aftertreatment system 2002 includes an optional light-offcatalytic system 2110, a TWC 2106, and an optional particulate filter2112.

If cylinder 2006 operates in a lean burn combustion mode, the combustionexhaust products are exhausted into the second aftertreatment system2004 via the exhaust valve 2104B and a second exhaust system 2022. Thesecond aftertreatment system 2004 may be configured for both lean burncombustion and stoichiometric combustion, including: (a) an optionallight off catalytic converter system 2110 (b) a TWC 2106 (c) anoxidizing catalyst 2114 (d) a NO_(x) trap and/or a Selective CatalyticReduction (SCR) system, (e) a particulate filter or any combination of(a) through (e). Not all the elements (a) thru (e) are necessary in theaftertreatment system. For example, if the second aftertreatment systemonly accepts combustion exhaust gases from LTGC, little NO_(x) will begenerated and a NO_(x) trap and SCR may not be necessary. In fact, insome situations no aftertreatment elements may be needed in the secondexhaust system 2022. It should be appreciated that the size and make-upof the various elements in the first and second aftertreatment systemsmay be different. For example, the TWC 2106 may be a different size inthe first and second aftertreatment system.

The optional light-off catalytic converter 2110 may be provided,upstream from other aftertreatment system elements in close proximity tothe internal combustion engine 2000. Since the light-off catalyticconverter 2110 is in close proximity to the engine 2000, it heats upquickly after a cold start. As a result, the light-off catalyticconverter 2110 is able to convert harmful exhaust gases into more benigngases following a cold start sooner than other aftertreatment elements.The light-off catalytic converter 2110; however, is typically smallerand has insufficient conversion capacity for sustained operation. As aresult, the other aftertreatment elements handle the majority of exhaustgas conversion once the engine and exhaust system heat up.

The optional particulate filter 2112 is provided to remove harmfulparticulates, such as soot, resulting as a byproduct of fuel combustion.In various embodiments, the particulate filter may be a wall flowfilter, a silicon carbide filter, a ceramic fiber filter, a metal fiberflow through filter, a paper filter, or any other type of filtersuitable for removing particulates.

With lean burn modes of operation, the resulting cylinder exhaust willinclude relatively high levels of oxygen (O₂). Conventional TWCs usedfor stoichiometric operation do not function very well in convertingnitrogen oxides to nitrogen in the presence of high levels of oxygen,since they rapidly become saturated with oxygen and will no longerreduce NO_(x). As a result, a NO_(x) trap or SCR is used in the secondaftertreatment system 2004 to reduce NO_(x) levels.

While FIG. 18 shows the first aftertreatment system 2002 as configuredfor stoichiometric combustion and the second aftertreatment system 2004configured for both stoichiometric and lean burn combustion it should beappreciated that other configurations are possible. For example, thesecond aftertreatment system 2004 may be configured solely for lean burncombustion. In this case the TWC 2106 may be removed.

Referring to FIG. 19, a flow diagram 1900 illustrating steps implementedby the engine controller 2102 for operating an internal combustionengine with two aftertreatment system is illustrated.

In step 1902, the engine controller 2102 selects the next cylinder inthe firing order of the internal combustion engine 2000.

In decision step 1904, the engine controller 2102 makes a decision tofire or not fire the next cylinder prior to the start of the nextworking cycle for the cylinder. The decision may be made on a firingopportunity by firing opportunity basis, although this is not arequirement.

With a skip decision, the flow chart returns to start and the nextcylinder in the firing order is selected in step 1902.

If a decision is made to fire the selected cylinder, then in decisionstep 1906, either stoichiometric or lean burn operation is selected. Inmaking this decision, the engine controller 2102 can use a wide range offactors, such as the current engine load or torque, engine speed, etc.Also, if some cylinders cannot operate in all modes, such as cases A, B,D, and E in table 1600 (see FIG. 16), then the decision taken must becompatible with the selected working cycle mode, i.e. stoichiometricburn or lean burn.

If the decision in step 1906 is a stoichiometric fire, then the flowchart 1900 proceeds to step 1908. In steps 1908 and 1910, the enginecontroller 2102 operates the cylinder in the stoichiometric mode andopens exhaust valve 2104A during the exhaust stroke of the workingcycle, while the other exhaust valve 2104B is closed. As a result, theexhaust is pass through the first aftertreatment system 2002 (see FIG.18).

Alternatively, if the decision in step 1906 is a lean burn fire, thenthe flow chart 1900 proceeds to step 1912. In steps 1912 and 1914, theengine controller 2102 operates the cylinder in the lean burn mode andopens exhaust valve 2104B during the exhaust stroke of the workingcycle, while the other valve 2104A is closed. As a result, the exhaustis pass through the second aftertreatment system 2004 as depicted in theFIG. 18 embodiment.

It should be also appreciated that any of the control operationsdescribed herein may be implemented using executable computer codestored in a suitable computer readable medium. The operations arecarried out when a processor executes the computer code. The computercode may be incorporated in an engine controller that executes dynamicfiring level modulation engine operation. The invention has beendescribed primarily in the context of gasoline powered, 4-stroke pistonengines suitable for use in motor vehicles. However, it should beappreciated that the described methods and apparatus are very wellsuited for use in a wide variety of internal combustion engines. Theseinclude engines for virtually any type of vehicle—including cars,trucks, boats, aircraft, motorcycles, scooters, etc.; and virtually anyother application that involves the firing of working chambers andutilizes an internal combustion engine. The various described approacheswork with engines that operate under a wide variety of differentthermodynamic cycles—including virtually any type of Otto cycle engines,Miller cycle engines, Atkinson cycle engines, Diesel cycle engines,Wankel engines and other types of rotary engines, hybrid engines, radialengines, etc. It is also believed that the described approaches willwork well with newly developed internal combustion engines regardless ofwhether they operate utilizing currently known, or later developedthermodynamic cycles.

Some of the above embodiments contemplate the deactivation of a workingchamber during skipped working cycles. In various implementations, thedeactivation of a working chamber involves preventing the pumping of airthrough the skipped working chamber during one or more selected skippedworking cycles. A working chamber may be skipped or deactivated in avariety of ways. In various approaches, a low pressure spring is formedin the working chamber i.e., after exhaust gases are released from theworking chamber in a prior working cycle, neither the intake valves northe exhaust valves are opened during a subsequent working cycle, thusforming a low pressure vacuum in the working chamber. In still otherembodiments, a high pressure spring is formed in the skipped workingchamber i.e., air and/or exhaust gases are prevented from escaping theworking chamber. The working chamber may be deactivated in any suitablemanner such that the working chamber contributes little or no net powerduring its power stroke.

Although only a few embodiments of the invention have been described indetail, it should be appreciated that the invention may be implementedin many other forms without departing from the spirit or scope of theinvention. For example, the control strategies described herein could beimplemented with a fully flexible valve trains that is not dependent ona camshaft for valve event timing. While the invention has generallybeen describe as using an intake and exhaust valve to control inductionand exhaust of a cylinder, a cylinder may have multiple intake and/orexhaust valves and the control strategies may collectively control theirmotion. While the invention has generally been described as usinggasoline as a fuel, many other types of fuel with gasoline-typecombustion qualities may be used either singly, as a mixture, or in adual fuel system with different fuels used on different types of workingcycles. Such fuels include, but are not limited to, hydrogen, ethanol,propanol, other alcohols, synthetic fuels, and natural gas. Therefore,the present embodiments should be considered illustrative and notrestrictive and the invention is not to be limited to the details givenherein.

What is claimed is:
 1. A method of controlling operation of an internalcombustion engine having a plurality of working chambers, the methodcomprising: operating the internal combustion engine in a skip fireoperational mode wherein one or more working cycles of the plurality ofworking chambers are either selectively fired or skipped; during somefired working cycles, using a lean air-fuel mixture and selectivelyoperating the associated working chambers in one of a Homogeneous ChargeCompression Ignition (HCCI) mode, a Gasoline Direct Compression Ignition(GDCI) mode, a Spark Assisted Compression Ignition Mode (SACI) mode, ora Dynamic Charge Compression Ignition (DCCI) mode; and during otherfired working cycles, selectively firing the associated working chambersusing a stoichiometric air-fuel mixture, and wherein the lean air-fuelmixture firings and the stoichiometric firings are intermixed.
 2. Themethod of claim 1, further comprising: ascertaining a torque demand forthe internal combustion engine; selecting the working cycles of theworking chambers that are skipped or fired based on at least in part onthe ascertained torque demand; and wherein a determination of whetherlean air-fuel mixture firings can be used is based at least in part onthe ascertained torque demand.
 3. The method of claim 1, wherein a rangeof torque values for which lean air-fuel mixture firings are used isexpanded when one or more of the working cycles of the working chambersare skipped relative to when all working cycles of the working chambersare fired.
 4. The method of claim 1, further comprising: ascertaining atorque demand for the internal combustion engine; determining if theascertained torque demand is below a threshold; and if the ascertainedtorque demand is below the threshold, selectively skipping the firing ofone or more work cycles of the working chambers so as to sustaincombustion of the other fired working cycles using the lean air fuelmixture.
 5. The method of claim 1, further comprising operating theinternal combustion engine in a dynamic skip fire mode where a decisionto either fire or skip each of the working chambers is made on a firingopportunity-by-firing opportunity basis.
 6. The method of claim 1,wherein for any given engine state with intermixed stoichiometric andlean air-fuel ratio working cycle firings, the stoichiometric workingcycle firings have a higher torque output than the working cycles firedusing a lean air-fuel ratio.
 7. The method of claim 1, furthercomprising promoting auto-ignition of the lean air-fuel mixture duringthe working cycles that utilize the lean air-fuel mixture by introducinghot exhaust gases into the fired working chambers.
 8. The method ofclaim 7, wherein the hot exhaust gases are introduced by one of thefollowing: (a) re-circulating of the hot exhaust gases from an exhaustsystem to the working chambers; (b) inducting the hot exhaust gases intothe working chambers by opening exhaust valves during intake strokes ofthe working chambers; (c) retaining residuals of the hot exhaust gasesin the working chambers from previously fired working cycles; or (d) anycombination of (a) through (c).
 9. The method of claim 1, furthercomprising selectively deactivating skipped working cycles of theworking chambers by (a) not fueling and (b) preventing air from pumpingthrough the working chambers during the select skipped working cycles.10. The method of claim 1, further comprising selectively allowing airto pump through, but not fueling, select skipped working cycles of theworking chambers.
 11. The method of claim 1, further comprising, astorque demands for the internal combustion engine vary, operating theworking chambers of the internal combustion engine by: (a) repeatedlydeciding to either skip or fire the working cycles of the workingchambers as needed to meet the varying torque demands; and (b) for thefired working cycles of the working chambers, repeatedly deciding to useeither the stoichiometric air-fuel mixture or the lean air-fuel mixturefor combustion, whereby outcomes of the repeated decisions (a) and (b)are made at least in part to satisfy the varying torque demands andimprove fuel economy, while providing acceptable levels of Noise,Vibration and Harshness (NVH).
 12. The method of claim 1, furthercomprising selectively increasing a temperature of exhaust gases fromthe internal combustion engine by increasing a number of the workingcycles that are fired using the stoichiometric mixture.
 13. A method ofcontrolling operation of an internal combustion engine having aplurality of working chambers, the method comprising: operating theinternal combustion engine in a skip fire operational mode wherein oneor more working cycles of the plurality of working chambers are eitherselectively fired or skipped; during some fired working cycles, using alean air-fuel mixture and selectively operating the associated workingchambers in one of a Homogeneous Charge Compression Ignition (HCCI)mode, a Gasoline Direct Compression Ignition (GDCI) mode, a SparkAssisted Compression Ignition Mode (SACI) mode, or a Dynamic ChargeCompression Ignition (DCCI) mode; and during other fired working cycles,selectively firing the associated working chambers using astoichiometric air-fuel mixture, and selectively decreasing atemperature of exhaust gases from the internal combustion engine byincreasing a number of the working cycles that are fired using the leanair-fuel mixture.
 14. The method of claim 1, further comprising passingexhaust gases resulting from combustion of the lean air-fuel mixturethrough one or more of the following aftertreatment systems: a light offcatalytic system; a three-way catalyst; an oxidizing catalyst; a NO_(x)trap; a Selective Catalytic Reduction (SCR) system; or a particulatefilter.
 15. The method of claim 1, further comprising passing exhaustgases resulting from combustion of the stoichiometric air-fuel mixturethrough one or more of the following aftertreatment systems: a light offcatalytic system; a three-way catalyst; or a particulate filter.
 16. Amethod of controlling operation of an internal combustion engine havinga plurality of working chambers, comprising: operating the internalcombustion engine in a skip fire operational mode in which one or moreworking cycles of the plurality of working chambers are eitherselectively fired or skipped; and for working chambers that are fired,during at least some of the fired working cycles, operating the workingchambers in one of a Homogeneous Charge Compression Ignition (HCCI)mode, a Gasoline Direct Compression Ignition (GDCI) mode, a SparkAssisted Compression Ignition Mode (SACI) mode, or a Dynamic ChargeCompression Ignition (DCCI) mode; and wherein a density of the skippedworking cycles is selected, at least in part to sustain combustion inone of the HCCI, GDCI, SACI or DCCI modes during at least some of thefired working cycles.
 17. The method of claim 16, further comprising forworking chambers that are fired during other working cycles, operatingthe working chambers in a stoichiometric mode.
 18. The method of claim17, further comprising deciding to individually operate the workingcycles of the working chambers in either the stoichiometric mode or oneof the HCCI, GDCI SACI, or DCCI modes based on factors including (a) atorque demand; (b) fuel consumption and (c) Noise, Vibration andHarshness (NVH).
 19. The method of claim 16, further comprising:ascertaining a torque request; determining if the ascertained torquerequest is within a range of torque values suitable for combustion usinga lean air-fuel mixture; and operating the fired working chambers in oneof the HCCI, GDCI SACI, or DCCI modes when the ascertained torquerequest is within the range of torque values suitable for combustionusing the lean air-fuel mixture.
 20. The method of claim 19, furthercomprising: operating other fired working cycles of the working chambersin a stoichiometric mode when the torque request is outside the range oftorque values suitable for combustion using the lean air-fuel mixture.21. The method of claim 19, wherein the range of torque values isexpanded when one or more of the working cycles of the working chambersis/are skipped relative to when all working cycles of the workingchambers are fired.
 22. A method of controlling operation of an internalcombustion engine having a plurality of working chambers, the methodcomprising: operating the internal combustion engine in a skip fireoperational mode in which one or more working cycles of the plurality ofworking chambers are selectively fired or skipped; and for workingchambers that are fired, during at least some of the fired workingcycles, operating the working chambers in one of a Homogeneous ChargeCompression Ignition (HCCI) mode, a Gasoline Direct Compression Ignition(GDCI) mode, a Spark Assisted Compression Ignition Mode (SACI) mode, ora Dynamic Charge Compression Ignition (DCCI) mode; ascertaining a torquedemand for the internal combustion engine; determining if theascertained torque demand is below a threshold; and if the ascertainedtorque demand is below the threshold, selectively skipping the firing ofone or more work cycles of the working chambers so as to sustaincombustion in one of the HCCI, GDCI, SACI or DCCI modes during otherfired work cycles.
 23. The method of claim 16, further comprisingoperating the internal combustion engine in a skip fire mode, whereinfor a given reduced effective displacement that is less than fulldisplacement of the internal combustion engine, at least one workingchamber is fired, skipped and either fired or skipped over successivefiring opportunities.
 24. The method of claim 16, further comprisingoperating the internal combustion engine in a dynamic skip fire modewhere a decision to either fire or skip the working chambers is made ona firing opportunity-by-firing opportunity basis.
 25. The method ofclaim 16, wherein for the working cycles of the working chambers thatare fired, selectively modulating torque output to be either a hightorque output or a low torque output.
 26. The method of claim 16 furthercomprising, during selected fired working cycles: providing a leanair-fuel mixture to an associated working chamber; compressing the leanair-fuel mixture in the working chamber; causing a triggered combustionevent within the working chamber while the air-fuel mixture iscompressed within the working chamber; and combusting the compressedlean air-fuel mixture within the working chamber in response to thetriggered combustion event.
 27. The method of 26, wherein causing thetriggered combustion event within the working chamber further comprises:direct injecting fuel into the working chamber; using a spark to ignitethe direct injected fuel.
 28. The method of 26, further comprising:ascertaining a torque load for the working chamber; determining if theascertained torque load is within a torque range for combustion of thelean air-fuel mixture; and introducing the lean air-fuel mixture intothe working chamber; and combusting the lean air-fuel mixture in theworking chamber during the working cycle.
 29. The method of claim 26,wherein the torque range is expanded when one or more of the workingcycles of the working chambers are skipped relative to when all workingcycles of the working chambers are fired.
 30. The method of 26, furthercomprising: ascertaining a torque load for a select working cycle of aselect working chamber; determining if the ascertained torque load isoutside of a torque range for the lean air-fuel mixture; and if theascertained torque is outside the torque range, introducing astoichiometric air-fuel mixture into the select working chamber; andcombusting the stoichiometric air-fuel mixture in the select workingchamber during the select working cycle.
 31. The method of claim 26,further comprising: ascertaining a torque demand for the internalcombustion engine; determining if the ascertained torque demand is belowa threshold; and if the ascertained torque demand is below thethreshold, selectively skipping the firing of one or more work cycles ofthe working chambers so as to sustain combustion of other fired workingcycles of the working chambers using the lean air-fuel mixture.
 32. Themethod of claim 26, further comprising operating the internal combustionengine in a skip fire mode, wherein for a given reduced effectivedisplacement that is less than full displacement of the internalcombustion engine, at least one working chamber is fired, skipped andeither fired or skipped over successive firing opportunities.
 33. Themethod of claim 26, further comprising operating the internal combustionengine in a dynamic skip fire mode wherein a decision to either fire orskip each of the working chambers is made on a firingopportunity-by-firing opportunity basis.
 34. The method of claim 26,wherein for the working cycles of the working chambers that are fired,selectively modulating torque output to be either a high torque outputor a low torque output.
 35. The method of claim 26, further comprisingoperating the internal combustion engine in a reduced displacement modewhere a first group of working chambers are continually fired and asecond group of working chambers are continually skipped for theduration of the internal combustion engine operating in the reduceddisplacement mode.
 36. The method of claim 16 wherein at least some ofthe fired working cycles are operated in the SACI mode.
 37. The methodof claim 16 wherein at least some of the fired working cycles areoperated in the HCCI mode.
 38. The method of claim 16 wherein at leastsome of the fired working cycles are operated in the GDCI mode.
 39. Themethod of claim 16 wherein at least some of the fired working cycles areoperated in the DCCI mode.